2, National Taipei University of Technology, Taipei, , Taiwan
Stretchable and deformable materials are nowadays important for the next-generation wearable electronics since they potentially can be biocompatible and possess distinct functionalities with our body movement. Stretchable devices, such as field-effect transistors, solar cells or memories based on polymeric materials have been demonstrated. However, it is still challenging to achieve stable and high electrical performance using an intrinsically-strained active layer. A semiconducting polymer system that possesses both superior electrical behavior and high stretchability, therefore, is needed.
Side chain engineering has been focused on the field of polymeric electronics very recently since rational side chain design can significantly manipulate the solution processability, solid state molecular stacking and thin film morphology. From the view of soft electronics, we aim to achieve a ductile active layer by controlling the inter-chain packing interaction and surface morphology based on the side chain substituents.
In this talk, two side chain design strategies on isoindigo-based conjugated backbone will be presented. Firstly, carbosilane side chains have been introduced with a simple synthetic pathway to evaluate long and branched side chains in high yields. A high mobility of 8.06 cm2V-1s-1 was probed using a top-contact transistor device based on carbosilane side chains consistent of a 8 carbon linear spacer plus two octyl chains after branching. More interestingly, such polymers possess a relatively low tensile modulus (< 0.4 GPa) and showed a stable mobility higher than 1 cm2V-1s-1 even under a 60% strain, which is one of the most successful example using a single conjugated polymer for soft and intrinsically stretchable transistor applications. Furthermore, the odd-even effect have been also explored for this carbosilane side chains system to understand the relationship between side chain length, thin film deformability, and stretchable device performance. Secondly, a low glass transition temperature (i.e. -54 oC) poly(butyl acrylate) (PBA) side chains has been incorporated to the conjugated polymer system. The soft PBA segment offers a great opportunity to improve the mechanical property of semiconducting polymer thin films that typically contain lots of rigid conjugated rings. As a result, this set of polymers exhibited superior thin film ductility with a low tensile modulus down to 0.12 GPa, and the mobility at 60% strain could be remained almost the same as non-stretched (i.e. 0% strain) condition. Based on our achievements, both side chain system showed not only good charge transport ability but also thin film ductility under intrinsically stretching, suggesting these newly-designed materials possessed great potential for next-generation wearable electronics.